U.S. Pat. No. 4,070,563 discloses what the applicant views as the most pertinent art to the present invention. It illustrates and describes a system of sewer flow monitoring wherein synchronized measurements of liquid level are made in pipes exiting into manholes, between which manholes flow in a pipe is to be measured. Prior approaches involved making liquid level measurements in the manholes. Because of non-uniformity of manholes and the lack of positive synchronization of measurements made at flow related manholes, the accuracy and reliability obtained was regarded by many as simply inadequate. In contrast, the system described in U.S. Pat. No. 4,070,563 has been widely heralded as superior and has been widely used during the past few years. In the patented system, liquid level measurements are precisely made within a pipe near its exit, these measurements being made by means of pressure sensing elements positioned in the bottom of the pipe. This type level measurement has proven quite successful as indicated. However, because of the environment, the pressure sensors being immersed in sewer fluid and sludge, it has been deemed desirable to provide a more durable system, one that could be located out of the sewage. With this in mind, a likely approach would be to employ sonic or ultrasonic type devices, such being rather widely used in the measurement of liquid levels in manholes in accordance with the prior art referred to above.
Typically in manhole liquid level measurement systems, sound transducers (one for transmission and one for reception, or a single transducer functioning as both transmitter and receiver) are typically positioned in the top region of a manhole well above anticipated liquid levels. A burst of ultrasonic energy is directed from the transmitting transducer downward to the surface of sewage in the manhole, and then, as an echo, the burst signal travels back up to the transducer functioning as a receiver. The transit time is detected to provide an indication of distance down to the level of the sewage.
Ideally, one would simply employ a manhole type ultrasonic measurement device for "in-pipe" measurements and thus overcome the problem described above. However, this simply will not work. First, existing equipment is not configured to fit into and function within a pipe. More importantly, however, it has been found that existing equipment simply will not make accurate measurements within a range wherein liquid levels are within a few inches of a receiving transducer. This difficulty arises from the fact that sonic energy directly received by a receiving transducer from a transmitting transducer tends to ring for some period of time after the transmitted pulse. Thus, there occurs a period immediately after the transmitted pulse when the effect of the transmitted pulse tends to mask and interfere with the detection of short range echoes returning during this interference period. Since a great many in-pipe measurements are to be made in relatively small sewage pipes, e.g., down to as little as eight inches, it is to be appreciated that several inches are lopped off of the short end of the range of measurement, and thus that such an approach is not feasible.
Another problem that often arises in sewage level measurement by ultrasonic means is that not infrequently echo signals will not follow a desired vertical path but will be deflected. Thus, for example, an echo may ricochet off the side wall of a pipe on its return travel to a receiving transducer. Alternately, the echo signal may, in its errant path, miss the receiving transducer and follow a path which involves multiple passes across the pipe before being received by the receiving transducer. Of course, in all cases where there is not a single echo and a direct pass back to the receiving transducer, an erroneous measurement of liquid level will occur.
With the foregoing in mind, it is the object of this invention to provide an improved flow monitoring system employing ultrasonic type liquid level measurements wherein the problem of short range measurements is overcome and wherein bad range signals are detected and ignored and, in general, a greater accuracy of measurement is achieved.